The purpose of this study is to develop a simple system for examining changes in microstructure (i.e., the orientational state and coil-stretch transition of polymer chains) of liquid polymers under extensional stress loading with the birefringence measurements.
For this purpose, we have chosen dripping-onto-substrate capillary breakup extensional rheometry (CaBER-DoS)[1]}, [2]}, [3]}, [4]}, [5]}, [6]} as the means of applying the extensional stress.
Unlike a rotational rheometer, which measures the shear viscosity, CaBER-DoS is a noncontact method that measures the extensional viscosity from the radius of the liquid filament, which decays as the fluid is extended.
In this paper, we report the results of an investigation of the chain morphology of a polymer chain in a stretched flow field, where elasticity becomes dominant.
Selection of the elastic-dominant regime, i.e., the elasto-capillary (EC) regime is particularly important when calculating viscoelastic properties such as the extensional viscosity, strain, and relaxation time.
Importantly, within the EC regime, a uniaxial extensional stress is applied to the fluid, allowing the assumption of uniform flow without shear stress to be made[4]}.
Therefore, the polymer chains inside the fluid are assumed to be oriented in the extensional direction and uniformly aligned, which should be experimentally demonstrable by the birefringence measurements.
Note that, according to the theory of CaBER-DoS, three regimes, dominated by inertia, elasticity, and viscosity, respectively, can be defined on the basis of the temporal decay of the radius of the liquid filament[3]}, [4]}, [10]}, [11]}, [12]}.
Thus, the most important requirement for our rheo-optical method is to be able to simultaneously measure the changes of the three regimes in CaBER-DoS and the birefringence, which reflects the morphology of the polymer chain.
To do this, a high-speed polarization camera, which enables us to obtain polarization information (i.e., the flow-induced birefringence and the orientation angle), is used.
